Sorption-desorption method and apparatus



OGL 23, 1962 N. D. coGGEsHALL ErAL 3,059,478

SORPTION-DESORPTION METHOD AND APPARATUS Filed Sept. 8, 1959 3 Sheets-Sheet 1 NITROGEN INVENTORS NORMAN D. COGGESHALL ORVILLE K. DOOLEN BY RALPH D. WYCKOFF ATTORNEY Oct. 23, 1962 N. D. coGGEsHALL E'AL SORPTION-DESORPTION METHOD AND APPARATUS Filed Sept. 8, 1959 3 Sheets-Sheet 2 ATTORNEY Oct. 23, 1962 N. D. COGGESHALL I'AL SORPTION-DESORPTION METHOD AND APPARATUS Filed Sept. 8, 1959 5 Sheets-Sheet 3 INVENTORS NORMAN D. COGGESHALL ORVILLE K. DOOLEN BY RALPH D. WYCKOFF ATTORNEY United stares Parent 3,059,478 SORPTN-DESRPTIOJ METHOD AND APPARATUS Norman D. Coggeshall, Verona, and Orville K. Doolen and Ralph D. Wyckoff, akmont, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa.,

a corporation of Delaware Filed Sept. 8, 1959, Ser. No. 838,553 12 Claims. (Cl. 73-432) The present invention relates to obtaining data characteristic of the physical surfaces of nely divided or porous solids, and more specifically the present invention is concerned with obtaining data having to do with the isotherms 'both on the sorption and desorption phases.

Broadly, the invention involves conducting the sorption phase in such a manner that equal mass increments are introduced at timed intervals, each of such timed intervals being of substantially suicient duration for establishment of pressure equilibrium. The timed intervals can all be equal throughout -the sorption phase but are not necessarily equal. The desorption phase is conducted in a similar manner yand preferably, though not necessarily, the mass increments withdrawn on desorption are equal to the mass increments introduced during the sorption phase. Also according to the invention, the pressure is recorded versus the total mass of gas introduced and removed during the sorption and desorption phases.

One broad aspect of the invention involves either introducing or removing from the sample volume equal mass increments of gas according to a predetermined timing schedule. Another aspect of the invention involves automatically continuing the introduction of equal increments of gas into the sample vessel until the pressure Within the sample vessel rises to a predetermined level, preferably remaining continuously at least at such pressure level for a predetermined time interval, and then automatically discontinuing the sorption phase and initiating the desorption phase by the incremental removal of equal masses of gas from the sample vessel.

In general, the aparatus of the invention for the transfer of equal mass increments of a gas from a first zone of relatively high pressure to a second zone of relatively low pressure, as the case may be for either the sorption or desorption phases, comprises a metering vessel having an inlet provided with a normally closed inlet valve for communication with the first zone and an outlet provided with a normally closed outlet for communication with the second zone, means for maintaining the vessel at a constant temperature, pressureresponsive electric switch means connected to the lvessel, said switch means including rst, second, and third contacts, with the iirst and second contacts being closed solely upon the pressure in the vessel exceeding a fixed, relatively hivh value and `with the iirst and third contacts being closed solely upon the pressure in the vessel exceeding a xed, relatively low value, a first electrical means connected to the switch means for opening the inlet valve solely during intervals initiated by the opening of the first and third contacts and terminated yby the closing of the iirst and second contacts, and a second electrical means connected to the switch means for opening the outlet valve solely during intervals initiated by the closing of the first and second contacts and terminated by the opening of the rst and third contacts. In the preferred construction, there is also provided a third electrical means including a control switch for delaying initiation of the interval of operation of the second electrical means until such control switch is actuated, together with timer means for actuating the control switch according to a predetermined timing schedule.

Substantially identical means such as described in the preceding paragraph is provided for introducing gas to the sample lvessel on one hand and for removing gas from the sample vessel on the other hand, such two means employing a common timing means and control switch; such combination additionally being provided with means for preventing concurrent operation of such two means and also for terminating operation of the means for introducing gas and simultaneously initiating operation of the means for removing gas from the sample vessel upon the pressure within the sample vessel rising to a predetermined value.

Another aspect of the invention involves an electromechanical pressure measuring system for measuring the pressure within the sample vessel in such a manner that the volume of gas in the Vessel is unchanged.

Basically, the pressure measuring apparatus according to the invention is a servo-operated differential pressure gauge wherein a gas pressure differential is measured by an arrangement involving a pair of bellows mechanically coupled back to back. The connection between the pair of bellows is connected to a sensitive balance employing a thin strip or ligament as a fulcrurn. A differential transformer is associated with the balance in such a manner as to actuate a servo-operated nulling system that operates through a mechanical linkage against the balance so as to maintain the juncture between the pair of bellows in a fixed position. The mechanical connection to the balance is coupled to a counter and to a potentiometric system associated with a recorder whereby the diierential pressure is recorded. The recorder is associated with the means for either introducing or removing gas from the samplev vessel in such a manner that the chart drive is actuated an equal increment of distance in a forward direction upon the occurrence of each increment of gas being either introduced or removed from the sample vessel.

Practice of the invention as broadly outlined above results in automatically producing in graphical form the manner in which the pressure within the sample vessel varies with the mass of gas in the sample vessel throughout both the sorption and the desorption phases. The graphical represen-tation thereby obtained is of interpretive value as to the surface characteristics of a sample undergoing analysis such as to pore size distribution, etc. The theory underlying the interpretation of such data is given in an article entitled The Determination of Pore Volume and Area Distributions in Porous Substances by E. P. Barrett, L. G. Joyner, and P. P. Halenda, I. Am. Chem. Soc., 73, 373 (1951). Further statements of theory underlying isotherm phenomena are given in U.S. Patent No. 2,692,497, issued October 26, 1954, to Van Nordstrand, and U.S. Patent No. 2,729,969, issued January 10, 1956, to Innes.

The invention will be better understood upon reference to the accompanying drawings illustrative of a preferred embodiment of the invention, wherein:

FIGURE l is a schematic illustration of the gas handling apparatus in general;

FIGURE :2 is a schematic diagram of the electromechanical system associated with the gas handling apparatus shown in FIGURE 1;

FIGURE 3 is a front elevational view of the difierential pressure measuring apparatus;

FIGURE 4 is a side elevational view of the ditierential pressure measuring apparatus shown in FIGURE 3; and

FIGURE 5 is a schematic illustration of the pressure measuring system shown in FIGURES 3l and 4.

Referring to FIGURE l, a sample vessel, which is small in size, is designated by `the numeral 10, such sample vessel being provided with a conduit 12, which is in turn provided with stopcocks 14 and 16. The conduit 12 is provided with a lateral Ibranch 18 intermediate the stopcocks 14 and 16 that is connected to a conduit 20 intermediate a pair of stopcocks 22 and 24 in the conduit 20. Disposed adjacent the sample vessel lil is a small vessel 26 that is connected by a conduit 28 to a diierential pressure measuring gauge designated generally at 30. The vessels and 26 are both disposed within a Dewar ask 32 containing liquid nitrogen 34. Such arrangement affords a constant temperature bath for the vessels 10 and 26 at the temperature of liquid nitrogen boiling at atmospheric pressure. 'I'he conduit 18 is provided with a branch conduit 36 communicating with the differential pressure measuring gauge 30, such conduit 36 being provided with a stopcock 38.

One end of the conduit 20 is connected to a nitrogen bottle 40 by a conduit 42. Normally closed, electrically actuated inlet and outlet valves 44 and 46, respectively, are interposed in the conduit 42. Throttle valves 48 and 50 are disposed respectively upstream and downstream of the valves 44 and 46 in the conduit 42.

A iacketed metering vessel 52 communicates with the conduit 42 intermediate the valves 44 and 46, as shown, and the vessel 52 is provided with connections 54 and 56 by means of which a tuid, such as water, can be circulated at a constant temperature so as to maintain the ljacketed vessel 52 and its contents at a substantially constant temperature.

A jacketed vessel S8 is provided which is connected to the vessel S2 by Way of a conduit 60 that includes a section 62 depending from the vessel 58. The upper end of the vessel 58 is connected to a short vertical conduit 64 that is provided with a normally closed, stopcock 66. The vessel 5S is provided with connections 68 and 70 through which a duid such as water can be circulated at a constant temperature to maintain the vessel S8 and its contents at a constant temperature. Disposed in the conduit 60 are three electrical contacts 72, 74, and 76 that are insulated from each other, and which are adapted to be electrically connected to each other by mercury in the conduit 60. The mercury in the conduit 60 is isolated from the vessel 5-2 by an enlarged chamber 78 in the conduit 60'. The conduit 60 is of small internal diameter so that only slight vertical movement of the mercury surface in the enlarged chamber 78 accompanies movement of the mercury surface from the contact 74 to the contact 76. The arrangement is such that a substantially fixed volume of a gas, such as air or nitrogen, trapped in the vessel 58 bears against one end of the mercury column in the conduit 60, while the gas pressure in the chamber 52 is applied against the other end of the mercury column in the conduit 60 lin enlarged chamber 78. It will be appreciated that two xed and relatively high and low gas pressures in the vessel S2 are required to place the surface of the mercury column in the conduit 60 at vertical positions just sufficiently high to contact the contacts 76 and 74. It will also be appreciated that a xed difference lin the mass of gas contained in the vessel 52 exists for any particular gas composition (nitrogen in this instance) when the pressure therein is just suicient for the mercury in the column 60 4to contact the contact 76 on one hand and contact 74 on the other hand; it being recalled that the vessel 52 is jacketed and a tiuid at constant temperature is circulated through the jacket. It is assumed of course that sufcient time is allowed for the gas content of vessel S2 to approach temperature equilibrium with the circulated fluid.

inasmuch as the gas volume in the conduit 42 intermediate the valves 44 and 46 and the connection to the vessel 52 is small, and as the gas volume in conduit 60 intermediate the vessel 52 and the enlarged chamber 78 is also very small compared to the volume of the vessel 52, the comments of the preceding paragraph as to the tixed difference in mass also apply with a high degree of accuracy when such additional volumes of gas are considered along with the volume of gas in the Vessel 52. In fact, it is the xed diierence in mass of gas existing in the latter instance that constitutes the amount of gas introduced as a iiXed and discreet increment into the sample vessel 10 through the conduits 42, 20, 18, and 12 during the adsorption or as it is sometimes called the absorption cycle or phase. If desired, heat exchange means, not shown, can be provided to maintain the volumes of gas additional to the volume within the vessel 52 at a constant temperature. This provision can generally be dispensed with in practice when room temperature is maintained at a relatively constant level and at a value approximately that of the circulating fluid.

In addition to the above-described apparatus for transferring equal mass increments of gas from the nitrogen supply 40 to the sample vessel 10, apparatus quite similar in principle is provided for withdrawing gas from the vessel 10 in an incremental fashion wherein the increments all have equal masses. This latter apparatus includes a conduit 80 connected to the conduit 20, wherein valves 82, 84, 86, and 88 are provided, these valves corresponding as to type and purpose to the previously described valves 44, 46, 48, and 50.

A metering vessel 90 basically similar to the vessel 52 is provided and connected to the conduit 80 intermediate the normally closed electric valves 82 and 84. The vessel 90 includes connections 92 and 94 through which a fluid at constant temperature can be circulated in the jacket portion thereof. A conduit 96 (corresponding to the conduit 60) is connected to the vessel 90, and the same is provided with electric contacts 98, 100, and 102 (corresponding to the contacts 72, 74, and 76) and an enlarged chamber 104 (corresponding lto the chamber 78). *Conduit 96 also contains mercury for the same purpose as the mercury in column 60. The upper end of the conduit 96 is connected to a vacuum pump, not shown, or a zone or xed low pressure to serve a purpose analogous to that previously described in connection with the vessel S8. The conduits 12 and 80 are also connected to vacuum, as shown, the lformer connection being for the purpose of initially evacuating the vessel 10 and the conduits connected thereto prior to conducting the adsorption (or absorption) phase, with the latter connection simply constituting a low pressure connection for discharging the vessel 90 to its lower pressure limit.

Attention is now directed to FIGURES 3, 4, and 5, wherein there is shown the means 30 for measuring pressure differential existing between the conduits 28 and 36. Secured to a wall member -106 by means of brackets 108 and 110 is a pair of bellows 1-'12 and '114. The bellows 112 and 114 are secured back -to back to a plate 116. As viewed in FIGUIRE 3, the right ends of the bracket 110 and of the plate 116 are connected by a thin strip or ligament 118, preferably metallic, as shown. Disposed below the bellows 112 and 114 is a lever arm 120. A fulcrum arrangement for the lever arm is provided that is constituted of a bracket 122 Secured to the wall 106, and a thin strip or ligament 124, preferably metallic, secured at one end yto the bracket and at the other end to an intermediate position along the length of the lever arm 120, as shown. A thin strip or ligament 126, preferably metallic, is secured at its opposite ends to the left ends of the plate 116 :and the lever arm 120, as shown in FIGURE 3.

As viewed in FIGURE 3, the right end of the lever arm 120 (which is preferably of a nonmagnetic metal such as aluminum) has xed thereto an armature 128, preferably of magnetically soft iron, that constitutes the movable part of `a dilferential transformer 130 otherwise fixed to the wall 106.

The conduits 28 and 36 are coupled to the iixed ends of the bellows 114 and 112, respectively, and communicate with the interiors thereof. With this arrangement, the action of the bellows 112 and 114 is to urge the plate 116 upwardly when the pressure in the conduit 28 exceeds that in the conduit 36, and vice versa. It will be understood that the bellows 112 and 114 are suciently exible about their central axes to allow transference of such forces through the ligament 126 to the lever arm 120.

Means is provided for applying a suilicient torque .to the lever arm 120 so as to maintain the plate 116 in fixed position. This torque is applied by a tension spring 132 applying an upward Iforce to the lever arm 120 intermediate the ligament 124 `and the armature 128. The neutral position to be maintained by the plate 116 is such that is occupied by the plate `116 when the pressures in the conduits 28 and 36 are equal, with sufcient tension being applied through the spring 132 to maintain al1 the ligaments 118, 124 and 126 under tension.

The means for applying the torque by way ofthe sprlng 132 will be more readily appreciated upon consideration of FIGURE 5. The plate 116 is shown as directly connected to the lever arm 120 for application of force thereto for movement about the schematically illustrated fulcrum constituted of the ligament 124. IMotion of the lever arm 120 caused by movement of the plate 1116 is communicated to the armature 128, with movement of the latter from its neutral position with respect to the yfixed portion of the diferential transformer 130 being sensed by the latter, with the result of an error signal being produced by the latter and transmitted to a servo ampliiier 134 by Way of electrical connecting means 136. The output of the servo amplifier 134 is fed -to a servo motor 138 by Way of electrical connecting means 140.

The servo arrangement is essentially conventional and functions to drive the motor 138 at a speed generally proportional to the displacement of the armature 128 from its neutral position, and in a direction dependent upon the direction of the displacement of the armature 128. Gearing means designated generally at 142 drives .a micrometer screw 144 from the servo motor 138 in a direction Such that the spring 132 attached to the screw 144 acts upon the lever arm 120 in a direction tending to return the armature 128 to its neutral position. The neutral position of the armature 128 corresponds to the neutral position of the plate 116 by virtue of the mechanical connections therebetween. Consequently, the servo system described above serves to continuously maintain the sysvtern in balance with the plate 116 in its neutral position. The tension applied to the lever arm 120 by the spring is linear With respect to the pressure differential between the conduits 28 and 36.

Since the spring 132 is essentially linear, the rotational position of the servo motor 138 and the gearing means 142 connected thereto at any instant is also linear with respect to the aforementioned pressure differential.

The gearing means 142 is mechanically coupled by means schematically illustrated at 146 and 148, respectively, to a command potentiometer 150 and a mechanical counter 152. The purpose of the potentiometer will be explained presently in connection with FIGURE 2.

Referring now to FIGURE 2, a recorder is designated generally at 154. Said recorder includes a movable chart 156 that is entrained over a drive roller 158 from a supply roller 160. A recording pen 162 is fixed to a continuous flexible wire or element 164 that is entrained over pulleys 166 and 168. Means is provided for driving the pulley 168 so as to move the recording pen 162 as a linear function of the extent that the servo motor 138 is rotated. The previously mentioned command potentiometer is provided for this purpose. The one terminal of the potentiometer 158 is connected in series to a terminal of a further potentiometer 170 through rheostats 172 and 174 by leads 176, 178, and 180. The other end terminal of the potentiometer 158 is connected to the other end 'terminal of the potentiometer 170 through a rheostat 182 by leads 184 and 186. A source of 6.3 volts alternating current 188 is provided that is connected to the leads and 186 by leads 198 and 192. The alternating current source 188 provides excitation for the potentiometers 158 and 170. The movable elements of the rheostats 174 and 182 are secured to a common control shaft 194, whereby such rheostats can be jointly adjusted by lthe single control knob 196. The rheostat 172 is provided with an adjustment knob 198 to be used in conjunction with the knob 196 in adjusting the overall system in a manner to be specified subsequently. The potentiometer 170 includes a `control shaft 200 lthat is connected to `a servo motor 282. The numeral 284 designates a servo amplier, one input connection being connected tothe center tap of the potentiometer 178 by a lead 206, and the other input connection being connected to the center tap of the potentiometer 150 by a lead 208. The output connections of .the servo amplifier 204 are connected to the servo motor 282 by leads 210 and 212. The numeral 214 designates a source of 110 volts alternating current which is connected to the servo ampliier 204 by leads 216 and 218, and to the servo motor 282 by leads 220 and 222. The servo motor 282 in addition to driving the potentiometer 170 also drives the pulley 168 by means of a shaft 224 and gearing 226.

The described electromechanical means for driving the pulley 168 from the servo motor 138 and its connection to the command potentiometer 150 is conventional and is known to those skilled in the art as a command followup system. It will be readily understood by those skilled in the art that the lcontrol knob 196 aiords a zero adjustment for the trace afforded by the recording pen 162, while the adjustment afforded by the knob 198 affords a provision for scale length adjustment.

The means for scheduling the operation of the valves 44, 46, 82, and 84, as well as the means for driving the chart will now be described. A source of direct current 228 is provided suitable for relay actuation. The negative or ground terminal 238 of the source 228 is connected by leads 232, 234, 236, 238, and 239 to the solenoids of relays 240, 242, 244, 246, and 247, respectively, from a common negative or ground lead 248.

The positive terminal 250 of the supply 228 is prov1ded with a principal lead 252. The relay 240 includes a contact 254, that normally closes a lead 256 between the contact 76 and the lead 252, but which closes a lead 258 between the contact 102 and lead 252 upon energization of the relay 240. The solenoid 241) includes another contact 260 that normally completes a connection between a lead 262 and a lead 264 connected to the contact 74, such `contact 260 completing a connection between the lead 262 and the lead 264 connected to the Contact 100 during energization of the solenoid 248.

The relay 242 includes a contact 268 that completes a `connection between rthe lead 262 and the lead 252 solely upon energization of the relay 242. The relay 242 in cludes a further contact 270 that normally connects the lead 252 to a lead 272, with such Contact 278 completing a connection between the lead 252 and a lead 274 upon energization lof the solenoid 242.

The relay 246 includes a contact 276 that normally connects a lead 278 to the lead 252, with such connection being broken upon the -relay 246 being energized.

The relay 247 includes a contact 280 that normally electrically connects the lead 252 to a lead 282, but which connects the lead 252 to a lead 284 upon energization of the solenoid 247.

The relay 244 includes a contact 286 that normally connects the lead 278 to the inlet valve 44, but which connects the lead 278 to the electric valve 82 upon energization of the relay 244. The relay 244 also includes a Contact 288 that normally connects the lead 282 to the electric valve 46, but which connects the lead 282 to the electric valve 84 upon energization of the relay 244.

The electric valves 44, 46, 82, and 84 in addition to their connections to the contacts 286 and 288 have their other terminals connected to the negative or ground lead 248 by leads 290, 292, 294, and 296, respectively.

The solenoid of the relay 242 is connected to the contacts "72 and 98 by a lead 298. The solenoids of the relays 246 and 247 are respectively connected to the leads 274 and 272. The solenoids 248 and 244 are arranged to be concurrently energized and can, in fact, be a single relay if desired in practice, such relays being shown as separate entities for the sake of simplicity in the drawings. The relays 240 and 244 are connected through a common lead 309 to a time delay relay 302 that is described more fully hereinafter.

A pair of electrical counters 304 and 306 are connected in parallel with the electric valves 86 and 46, as shown, so that individual counts of the number of times that each of such valves is opened is obtained.

The time delay relay 302 is of a conventional type such that continuous closure of a normally open micro switch 308 for a predetermined time completes a circuit between the leads 252 and 300 'so as to energize the relays 240 and 244. A source of 110 volts alternating current 310 is connected to the time delay 302 by leads 312 and 314, the micro switch 303 being disposed in series in the lead 314. As stated previously, the time delay relay 302 is conventional and can be of the type manufactured by A. W. Haydon Company of Waterbury, Connecticut bearing manufacturers No. Lll4l9.

Attention is now directed to the means for advancing the chart '156 an equal increment of distance upon each occasion of either of the valves 46 or 84 being opened. Such means comprises a solenoid i316 connected to the leads 282 and 248, it being apparent that the solenoid 316 is energized whenever the relay 247 is not energized. An armature 318 is associated with the solenoid 316 so that the former is depressed from a normally raised position during the energization of the solenoid 316. The lower extremity of the armature 318 is provided with a pivoted pawl 320 that engages with a tooth of a ratchet Wheel 322 mounted on the shaft of the roller 158 during downward movement of the armature 318. The arrangement is such that each successive energization of the solenoid 316 results in the roller 158 being rotated a xed amount in a yforward direction so as to advance the chart 156 in a corresponding manner.

As ldescribed thus far, the chart 156 is driven by incremental jumps so that yvarying dilerential pressure in the conduits 28 and 36 will record an inked line or trace of the nature indicated at 324. The general form of the inked line 324 made by the recording pen 162 is explained fully later. Y

lFor the purpose of timing the introduction into or the removal of equal mass increments from the vessel 10, there is provided an electric timing motor 326 having power connections, not shown. An endless chain 32S is entrained over a sprocket Wheel 330 driven by the electric timing motor 326. The endless chain 328 is provided at spaced intervals along its length with laterally extending pins or extensions 332. Disposed along the travel path of the chain 328 is a normally closed micro switch '334, the arrangement being such that micro switch 334 is engaged by and opened during the time interval that each of the pins 332 travels by the micro switch 334. Hence, the micro switch 334 can be intermittently opened according to any predetermined timing schedule desired dependent upon the spacing of the pins 332 on the chain 328. Of course, a number of chains can 'be provided of various lengths and a variety of pin spacings (either regular or irregular) thereon that can be interchangeable on the sprocket wheel 330 to aord swift and convenient versatility in adjusting timing schedules. The micro switch 334 is connected between the leads 284 and 272.

The operation of the illustrated apparatus will now be described. Initially, with a sample of material to be analyzed in the vessel 18, the valves 44, 46, 82, and 84 are closed and the stopcocks 14, 16, 22, and 24 are in their customarily closed positions. The stopcocks 16 is opened and the system is evacuated after which the stopcock 16 is closed. The conduit 2S contains nitrogen gas and the vessel 26 contains liquid nitrogen, so that the pressure in the conduit 28 is that of Vthe vapor pressure of liquid nitrogen at the temperature prevailing 4in the sample vessel 19. It is assumed that the interior of the conduit 42 and the vessel `52 have previously been ilushed free of all other gases than nitrogen. The introduction of increments of nitrogen gas into the vessel 10 is now commenced and is accomplished by opening the Valve 44 until sufficient nitrogen gas enters the vessel 52 that the mercury in the column '60 makes contact with the contact 76. The throttling valve 48 limits the rate of gas entering the vessel 52 to prevent pressure surges. Then the valve 44 is closed after which the valve 46 is opened and gas allowed to leave the vessel 52 until the pressure in the vessel S2 falls to such a value that the mercury just breaks Contact with contact 74 at which time the valve 46 is closed. The throttling valve 50 prevents too rapid gas movement and any harmful pressure surges. As described previously, such action results in the delivery of `a fixed mass of nitrogen from the vessel 52. This operation of the valves 44 and 46 is then repeated in a cyclic manner to the conclusion of the adsorption phase of the analysis.

The operation of the valves 44 and 46 is performed automatically by the apparatus shown in 'FIGURE 2. The description of such automatic operation will be most readily understood if it is assumed initially that the vessel 52 is filled to the extent that the mercury contacts the contact 76. lt =will be fur-ther assumed that the relays 240 and 244 are fle-energized and that none of lthe pins 332 are engaging the micro switch 334 so as to open the latter. Also, it is assumed that the relay 247 is energized through the contact 288 and the switch 334. Under these circumstances, the relay 242 is energized through the contact 254, the contact 76, the mercury in the conduit 60, the contact 72, and the lead 298.

As both the relays 246 and 247 are energized, the contacts 276 and 280 open the circuit to the valves 44 and 46 so that they are de-energized and closed. It will be noted that valves 82 and 84 cannot be energized unless relay 244 is energized. This situation will continue until such time as the switch 334 is actuated open by one of the pins 332 which deenergizes relay 247, with the result that contact 280 moves to close the circuit to the valve 46, opening the latter to permit discharge of gas from the vessel 52.

This situation will continue -until the mercury breaks contact with the Contact 74, it being noted that relay 242 is kept energized during this interval by the contact 268, contact 26) and contact 74. However, upon the mercury breaking contact with the contact 74, the circuit holding the relay 242 energized is broken and the relay 242 is de-energized, whereupon the contact 27|]Y moves to a position energizing the relay 247 and de-energiz-ing relay 246. Energization of the relay 247 de-energizes the valve 46 closing the latter, while de-energization of relay 246 allows the contact 27 6 to complete a circuit affording energization of the valve 44 opening the latter. It should be noted at this time that the relays 246 and 247 both are of the character that move the contacts controlled thereby swiftly on energization but allow such contacts to return to their normal positions more slowly upon de-energization. This prevents the valves 44 and 46 being concurrently open.

This situation will continue until the mercury again makes contact with the contact 76. It will be observed that when the relay 247 is energized with the micro switch 334 closed, the relay is held energized by the contact 280 until such time as the micro switch 334 is opened by one of the pins 332. Upon the mercury contacting the contact 76, the relay 242 is energized and the apparatus has returned insofar as an adsorption dosing cycle is concerned to the initially assumed set of conditions, whereupon a subsequent dosing cycle will be initiated by one of the pins 332 opening the micro switch 334. It will be appreciated that the time intervals between actuations of the micro switch 334 are adjusted so as to be longer than the time required for the vessel 52 to discharge and then be recharged. Consequently, the vessel 52 is in readiness to be discharged upon each opening of the micro switch 334.

During the period of time that the vessel 5.2 and the valves 44 and 46 are metering increments of gas to the vessel 10, the chart is being advanced a xed amount at the beginning of each discharge of the vessel 52 through the action of the previously described solenoid 316, pawl 320, and ratchet wheel 322 arrangement. Also, the recording pen 162 constantly occupies a position transverse to the travel path of the chart 156 that is linear with respect to the pressure differential existing between the conduits 28 and 36. This latter function is achieved by virtue of the previously described command follow-up system and the pressure differential measuring apparatus. Eventually, as the adsorption phase is continued, the pressure dierential reaches a predetermined value at which the adsorption phase is to be discontinued. Such predetermined pressure differential corresponds to a certain position of the recorder pen 162, and the micro switch 368 is arranged to be engaged by the pen 162 or its mount when the pen occupies such certain Iposition. Inasmuch as the differential pressure may reach `and thence withdraw from the predetermined value several times before it is safe to conclude that the adsorption phase should be terminated, there is provision for automatically terminating the adsorption phase and initiating the desorption phase solely after the differential pressure has reached the predetermined value Without withdrawal for a predetermined time. Actuation of the micro switch 308 to its closed position for the predetermined uninterrupted time interval by the pen 162 or its mount completes a circuit between leads 252 and 300 so `as to energize the relays 246 and 244. Such circuit remains closed until manually reopened.

Energization of the relays 246 and 244 terminates the adsorption phase by moving the contacts 286 and 288 so that the valves 44 and 46 cannot be energized open, but so that valves 82 and 84 can respectively be energized open in their stead upon the relays 246 and 247 respectively being de-energized. Energization of the relay 240 moves the contacts 254 and 260l so that contacts 160 and 102 electrically assume the role previously played by the contacts '74 and 76, respectively, it being noted that contacts 72 and 98 are directly connected. Therefore, it will be seen that energization of the relays 240 and 244 effectively substitutes the elements 82, 84, 98, 100, and 102 for the elements 44, 46, 72, 74, and 76 electrically so that the valves S2 and 84 are now cyclically operated in an analogous manner to the previously described operation of the valves 44 and 46. Such change in mode of operation results in the metering vessel 90 exhausting equal masses of nitrogen from the sample vessel upon the occurrence of each instance of one of the pins 332 open 10 ing the micro switch 334. valves S6 and 88 serve the same functions during the desorption phase as the valves 48 and 50 serve during the adsorption phase, and that the time consumed for the vessel 9i) to discharge and to be charged again is less than the time interval between successive openings of the micro switch 334 by the pins 332.

From the foregoing it will be evident that the recorder 154 operates to produce a continuous record of the adsorption phase followed by the desorption phase wherein the trace 324 plots the differential pressure versus mass of gas either introduced or removed. It will be noted that the lmass scale of each phase will be the same if the apparatus is adjusted so that the mass increments discharged from the vessel 16 (each of which is, of course, equal to each other) are each equal to the mass increments in troduced to the vessel 10 (each of which is, of course, equal to each other) during the adsorption phase.

The `trace or record 324 closely approximates the continuous equilibrium isothermcurve and furnishes the basic data -for calculating the physical surface characteristics of the sample analyzed. Of course, the mode of chart drive results in the Itrace 324 being irregular or steplike. It is contemplated that the char-t 156 can be, as is conventional in recorders, driven at a constant rate; however, the illustrated mode of chart drive is preferred in the interest of minimizing the length of the record obtained and for the reason that positions at which the outlet valves 46 and y84 are opened are very plainly marked; also, the transverse portions of the trace 324 made while lthe chart 156 is not moving afford an easily interpreted measure of the extent that the pressure in the vessel 10 changes in approaching equilibrium pressure for each introduction or removal of an increment `of nitrogen. Such measure of equilibrium pressure adjustment is also of interest' with respect to the average slope of the trace 324 and can serve as a guide to the sutdciency of the spacing of the pins 322 for the trace 324 to closely approximate lthe true isotherm.

It is believed that the lforegoing is amply sufcient to afford a |full and complete understanding of the principles of the invention. Obviously, the apparatus for employing the principles of the invention is susceptible to numerous variations Without departing from the spirit of the invention, and therefore the actual scope of the invention should be ascertained upon inspection of the appended claims.

We claim:

l. The method of producing isotherm data with -respect to surface characteristics of a solid sample cornprising, placing the sample in an evacuated zone, maintaining the zone at 'the temperature of liquid nitrogen, periodically introducin-g equal mass increments of nitrogen into the zone, the introduction of the increments be-v ing sufficiently spaced apart in time for the nitrogen to closely approach lequilibrium pressure in the zone, exposing the pressure in the Zone and the vapor pressure of liquid nitrogen at the same temperature as the zone, respectively, -to opposing sides of a pressure differential measuring means to measure the pressure diiferential, and recording such pressure differential versus the mass of nitrogen introduced into the zone.

2. The method of claim 1, wherein the differential pressure is continuously recorded versus the number of increments of nitrogen introduced, whereby the differenltial pressure immediately following introduction of a nitrogen increment can be compared directly with the resulting equilibrium pressure attained.

3. The method of producing isotherm data with respect to surface characteristics of a solid sample comprising, placing the sample in an evacuated zone, maintaining the zone at the temperature of liquid nitrogen,- periodically introducing equal mass increments of nitrogen `into the zone, .the introduction of the increments being sufliciently spaced apart in time for the nitrogen to Itis to be understood that the closely approach equilibrium pressure in the zone, after a predetermined equilibrium pressure in the zone has been attained, periodically removing equal mass increments of nitrogen from the zone, the removal of the increments being sufficiently spaced apart in time for the nitrogen to closely approach equilibrium pressure in the zone, exposing the pressure in the zone and the vapor pressure of liquid nitrogen at the same temperature as the zone respectively, to opposing sides of a differential pressure measuring means to measure the pressure differential, and recording such differential pressure versus the number of increments of nitrogen that have been introduced and removed from the zone.

4. Apparatus for the transfer of equal mass increments of a gas Afrom a first zone of relatively high pressure to a second zone of relatively low pressure comprising a metering vessel having an inlet provided with a normally closed inlet valve for communication with the first zone and :an outlet provided with a normally closed outlet valve for communication with the second zone, means for maintaining 4the vessel at a constant temperature, pressure responsive electric switch means connected to the vessel, said switch means including first, second and third contacts with the first and second contacts being closed solely upon the pressure in the vessel exceeding a fixed relatively high value and with the first and third contacts being closed solely upon the pressure in the vessel exceeding a fixed relatively low volume, a first electrical means connected to the switch means for opening the inlet valve solely during intervals initiated by the opening of the first and third contacts and terminated by the closing of the first and second contacts, and a second electrical means connected to the switch means for opening the outlet valve solely during intervals initiated by the closing ofthe first and second contacts and terminated -by the opening of the first and the third confacts.

5. Apparatus for the transfer of equal mass increments of a gas from a first zone of relatively high pressure to a second zone of relatively low pressure comprising a metering vessel having an inlet provided with a normally closed inlet valve for communication with the first zone and an outlet provided with a normally closed outlet valve for communication with the second zone, means for maintaining the vessel at a constant temperature, pressure responsive electric switch means connected to the vessel, said switch means including first, second and third contacts with the first and second contacts being closed solely upon the pressure in the vessel exceeding a fixed relatively high value and with the first and third contacts being closed solely upon the pressure in the vessel exceeding a fixed relatively low value, a rst electrical means connected to the switch means for opening the inlet valve solely during intervals initiated by the opening of the first and third contacts and terminated by the closing of the first and second contacts, a second electrical means connected to the switch means for opening the outlet valve solely during intervals initiated by the closing of the first and second contacts and terminated Iby the opening of the rst and the third contacts, and a third electrical means including a control switch for delaying initiation of the interval of operation of the second electrical means until such control switch is actuated.

6. The combination of claim 5 including timer means for successively actuating .the control switch according to a predetermined time schedule.

7. Apparatus for the transfer of equal mass increments of a gas from a first zone of relatively high pressure to a second zone of relatively low .pressure comprising a metering vessel having an inlet for communication with the first zone and an outlet for communication with the second zone, said inlet and said outlet being provided with normally closed inlet and outlet valves respectively, means for maintaining the vessel at a constant temperature, pressure-responsive electric switch means connected to 12 said vessel, said switch means including first, second and third contacts, with the first and second contacts being closed solely upon pressure within the vessel being in excess of a fixed relatively high pressure in the vessel and with the first and third contacts being closed solely upon pressure within the vessel being in excess of a fixed relatively low pressure, a first locking relay means operatively connected to the pressure switch means for energization during closure of the first and second contacts and for remaining energized during uninterrupted closure of the rst and third contacts, said means connected to the first locking relay means for opening the inlet valve solely during intervals that the first locking relay means is de-energized, a normally closed control switch, a seci ond locking relay means operatively connected to the first locking relay means for energization during deenergization of the first locking relay means and remaining energized during uninterrupted closure of the control switch, and electrical means for opening the outlet valve solely during intervals that the second locking relay means is deenergized, whereby the increments can `be ltransferred according to a predetermined timing schedule.

8. The combination of claim 7 including means for measuring the pressure in one of said zones in relation to a reference standard, and means for recording the measured pressure versus the number of increments transferred.

9. The combination of claim 8, wherein said reference standard is a gas under pressure, said measuring means comprising a pair of bellows arranged back to back with their opposite ends being fixed and communicating with the reference standard gas pressure and the said one zone, rst and second ligaments each having one end connected to the juncture of the bellows, with such connections being spaced apart, the first ligament having the remote end thereof fixed, a lever arm and a third ligament constituting a fulcrum for such arm, said second ligament having the end thereof remote from the bellows juncture secured to the lever arm, a differential transformer arranged to sense displacement of the lever arm from a neutral position thereof, a tension spring connected to the lever arm and a micrometer screw connected to the spring for varying the tension of the spring, a servo system coupled to the transformer for driving the micrometer screw in a direction that the tension of the spring returns the lever arm to its neutral position, said servo system including a servo motor, said recording means including a command follow-up operatively coupled to the servo motor.

10. The combination of claim 9', wherein the recording means includes a chart and an intermittent driving means therefor that translates the chart a unit distance at the initiation of each increment transfer.

ll. Differential gas pressure measuring means comprising a pair of bellows arranged back to back with their opposite ends being fixed and communicating with the gas pressures, the differential pressure between which is to be measured, first and second ligaments each having one end connected to the juncture of the bellows, with such connections being spaced apart, the first ligament having the remote end thereof fixed, a lever arm and a third ligament constituting a fulcrum for such arm, said second ligament having the end thereof remote from the bellows juncture secured to the lever arm, a differential transformer arranged to sense displacement of the lever arm from a neutral position thereof, a tension spring connected to the lever arm and a micrometer screw connected to the spring for varying the tension of the spring, la servo system coupled to the transformer for driving the micrometer screw in a direction that the tension of the spring returns the lever arm to its neutral position, said servo system including a servo motor, means for recording rorational displacement of the servo motor as a measure of the -pressure differential including a command follow-up system coupled to the servo motor.

12. Apparatus comprising a sample vessel, first and Mrk second conduits connecting the sample vessel to a gas source and sample vessel evacuating means respectively, means in each of said conduits for transferring equal mass increments of a gas from a rst zone of relatively high pressure to a second zone of relatively low pressure, means for preventing concurrent operation of the means in the rst conduit and the means in the second conduit, means for periodically causing the operative one of the means in the rst and second conduits to transfer a mass of gas, means for terminating operation of the means in the rst conduit and also for initiating operation of the means in the second conduit upon the pressure in the sample vessel rising to a predetermined level, and means for recording the pressure in the sample vessel versus the number of gas transfers.

References Cited in the le of this patent UNITED STATES PATENTS 2,692,497 Von Nordstrand Oct. 26, 1954 2,694,927 Coulbourn et al Nov. 23, 1954 2,729,969 Innes Jan. 10, 1956 2,816,443 Gomez et al. Dec. 17, 1957 2,910,870 Schaefer Nov. 3, 1959 OTHER REFERENCES Schlosser et al.: German application, 1,057,798, printed May 21, 1959. 

